1. Field of the Invention
The present invention is directed to a method of calibrating an analytical tool, and more specifically, to a method of calibrating electron microscopes for precise calibration of low to medium magnification ranges.
2. Description of Related Art
Electron microscopes, such as, transmission electron microscopes (TEM), are commonly used in the process of fabricating integrated circuits. TEMs are used to microscopically examine portions of a semiconductor die to determine the results of new or conventional processes. The examination may be to confirm the results of an experimental process, to determine the nature of a particular failure or defect in a semiconductor device, or even to find impurities within the semiconductor device. Of course, because of the nature of integrated circuits, the examination must often be performed on samples cut from the die in question.
The step of examining a semiconductor wafer for defects and structures is crucial in semiconductor fabrication as certain defects typically cause semiconductor device failure. In examining the semiconductor wafer using TEM, the wafer is removed from the production line and brought to an analytical tool for analysis. However, prior to inspection, the TEM tool must be calibrated to accurately and effectively inspect the semiconductor wafer for microstructural information.
Calibration of a TEM tool can be accomplished by a variety of known techniques. The most common calibration techniques for TEM processing include in-situ calibration and permanent calibration.
In-situ TEM calibration techniques require having a feature on the sample to be analyzed whereby the size and/or geometry of such feature are precisely known. As the exact feature size to be analyzed must be known, in-situ calibration is typically used for high magnifications, i.e., those magnifications greater than 200,000×, where the crystalline lattice spacings can be imaged. In those low to medium magnifications, i.e., those having magnifications ranging from 5000× to 200,000×, in-situ calibration is rarely used as the features to be analyzed by TEM are significantly smaller, and as such, the exact sizes and/or geometries of such features are not exactly known.
With low to medium magnifications, permanent TEM calibration techniques are typically performed on the TEM tool. In so doing, these low to medium magnifications of the tool are calibrated using “standards” having features of known sizes, geometries and/or thicknesses, either in plan view or cross sectional view. However, a disadvantage of permanently calibrating the TEM tool using standards is that such standards must have a wide range of these known features that match the fields of view for the magnification settings in question. A further disadvantage is that the standards typically must be removed from the TEM tool and rotated for calibration of such TEM tool in the X and Y directions. Another disadvantage of permanent calibration techniques is that they require measuring feature edges of the sample be analyzed, which, are often ill defined and introduce error into the calibration. Thus, as the edges of current and future generations of semiconductors continue to diminish in size, permanent calibration is inefficient as medium to high range magnification settings are required for analyzing these smaller edges.
Another permanent TEM calibration technique includes superimposing two crystalline materials, having known lattice spacings, to derive a Moire fringe pattern. This pattern is then used as a calibration “ruler.” This technique enables the use of the entire field of view for calibration, yet, it only provides precise calibration for magnification settings whose field of view encompasses a significant number of Moire fringes, which is dependent on the two crystalline materials chosen. Further, the relative orientation between the two materials must be exactly known so that the Moire fringe spacing can be analyzed to high precision. As such, a variety of different precisely oriented crystalline pairs must be used to calibrate the entire magnification range from 5,000× to 200,000×.
It would be advantageous to have an analytical calibration technique that calibrates, to a high precision, a variety of magnifications over a wide range, particularly magnifications ranging from 5,000× to 200,000×, using a single calibration standard.
Therefore, a need continues to exist in the art for improved techniques and systems for calibrating analytical tools, particularly electron microscopy tools, which enable the use of a single sample to determine magnification over a medium to high magnification range, particularly 5,000× to 200,000×, in addition to providing a “ruler” which fills the entire field of view for each of a variety of selected magnification settings on such single sample.